Corrosion resistant ceramic media

ABSTRACT

The present invention relates to novel ceramic media or ceramic media coatings comprising mainly magnesia, silicon dioxide, and alumina, with forsterite and spinel as the dominant crystalline phases, which show high resistance to alkali attack at high temperature. Ceramic materials having these characteristics are particularly well suited for use as heat-exchange media in regenerative thermal oxidizers (RTOs) for the wood process industry.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional PatentApplication No. 60/372,953 filed on Apr. 16, 2002.

FIELD OF THE INVENTION

[0002] This invention relates to ceramic regenerative heat transfermedia for use in Regenerative Thermal Oxidizers.

BACKGROUND OF THE INVENTION

[0003] Conventional ceramic heat-transfer media used in RTOs typicallyconsist of 70˜75% SiO₂, 20˜25% Al₂O₃, 2˜5% K₂O or Na₂O, and traceamounts of Fe, Ca, and Ti. The media can be shaped like honeycombs,plates, saddles, or other forms. When ceramic media having suchcomposition are used in an alkaline environment, reactions will occurbetween the surface of the media and the alkaline component.

[0004] As a result, a layer of reaction products builds up on thesurface of the media, which increases in thickness over time such thatthe effective void fraction of the media is decreased. The reduction invoid fraction eventually reaches a point at which the increased pressuredrop of gas flowing through the media impairs the operating efficiencyand performance of the equipment, which then has to be shut down topermit replacement of the media. This not only adds to the cost ofmedia, but it may also result in lost production.

[0005] Alkali-resistant ceramics used in the metal finishing and glassindustries contains silicon carbide, silicon nitride, alumina, zirconia,and similar materials that have to be formed under high pressure. As aresult, these ceramics are quite expensive. More importantly, theseceramic compositions cannot be shaped using ordinary methods, so theyoften cannot be made into the same shapes as conventional ceramic media.

[0006] U.S. Pat. No. 5,731,250 to Reid et al. teaches the use ofzircon-based ceramic bodies made from a composition that can be formedby conventional processes. However, zircon is expensive, and it would beuseful to make heat-exchange media from a material with even betterresistance to alkali attack.

[0007] As is well known, regenerative thermal oxidizers (RTO) usuallyconsist of two or more heat-exchange canisters with one combustionchamber. Heat-transfer media are installed in the heat-exchangecanisters in order to store and release heat.

[0008] In the wood process industry, the waste gas treated by RTOs oftencontains substantial amounts of solid particulates. Analysis by scanningelectron microscopy (SEM) indicates that the residue left on the mediaby partial oxidation of such solid particulates consists of: TABLE 1Composition of char/ash residue from wood dust Component C O Na Mg Al SiS K Ca Percent 68.56 27.71 0.83 0.71 0.36 0.5 0.5 0.44 0.37

[0009] Additional analysis using Auger electron spectroscopy (AES) showsthat wood ash contains large amounts of Na₂SO₄, K₂CO₃, CaCO₃, MgO,Al₂O₃, SiO₂ and other inorganic compounds. At the operating temperatureof an RTO (850° C.), these components can become corrosive. Thecorrosive nature of these compounds can cause chemical reactions withordinary porcelain or stoneware. These reactions can reduce the voidfraction of the media and impair the performance of the RTO.

[0010] Analysis of media that had been installed in an RTO for tenmonths reveals that the surface layer of the media contained the samecomponents as wood ash, indicating that the wood ash has reacted withthat layer of ceramic material. TABLE 2 Surface composition of chemicalporcelain media after 10 months in RTO Component C O Na Mg Al Si S K CaPercent 20.34 42.88 15.12 0.7 7.57 9.59 3.17 0.33 0.7

[0011] Analysis by AES and X-ray diffraction spectroscopy shows that thesurface layer consists of sodium aluminosilicate (Na₂AlSiO₄), potassiumaluminosilicate (K₂AlSiO₄), and various other compounds of potassium,calcium, aluminum, silicon and/or sulfur. These are the reactionproducts that cause problems with the media.

[0012] There is therefore a need for an economical alkali-resistantceramic material that can solve the problem of chemical attack by hotalkali in RTOs.

DESCRIPTION OF THE INVENTION

[0013] The present invention provides an alkali-resistant materialcomprising 20˜80 wt % MgO, 10˜50 wt % SiO₂, 5˜30 wt % Al₂O₃, and 1˜10 wt% Fe₂O₃, CaO, Ka₂O and/or Na₂O, with forsterite and spinel being thedominant crystalline phases.

[0014] The MgO may be derived from oxides of magnesium or from talc. TheSiO₂ and Al₉O₃ may be derived from clay.

[0015] The alkali-resistant material in accordance with the presentinvention can be formed in the same way as conventional ceramic media.The raw materials are ground up into particles, 80% of which are smallerthan 50 microns and mixed with water to form a paste which can beextruded or pressed into shape, then dried and fired at 1,250° C.˜1,450°C.

[0016] This alkali-resistant material can also be applied as a surfacecoating to conventional ceramic media shapes. The coated product is thenfired at 1,250° C.˜1,450° C. Among the chemical reactions that may occurare the following:

Al₂O₃+MgO→MgAl₂O₄

Al₂O₃+2 MgO→Mg₂Al₂O₅

2 MgO+SiO₂→Mg₂SiO₄

[0017] After firing, the material has porosity less than 5% by volume,water absorption less than 5% by weight, and compressive strengthgreater than 2×10⁸ N/m². Analysis by x-ray diffraction spectroscopyreveals the characteristics peaks of forsterite and spinel, indicatingthat these are the predominant crystalline phases. There is no evidenceof any quartz phase.

[0018] The ceramic material in accordance with the present invention,when used at temperatures between 200˜1,100° C., shows high resistanceto alkali attack.

EXAMPLE 1

[0019] Ceramic saddles were prepared from the alkali-resistant materialin accordance with present invention. The alkali-resistant material wasprepared from the following raw materials: Alkali-Resistant CeramicMaterial Chemical Composition of the Raw Materials Raw Materials (weight%) (weight %) Roasted Magnesia 80 MgO 76.3 Ceramic Clay 14 SiO₂ 13.6Limestone 5 Al₂O₃  5.2 Water Glass 1.0 CaO  2.9 Carboxymethyl Cellulose1.5 K₂O  0.4 Na₂O  1.4 Fe₂O₂  0.2

[0020] The raw materials listed above were ground in the dry state to aparticle size less than 50 microns for 80% of the particle. Water isthen mixed into the ground material to make a homogeneous paste. Apressure filter was then used to remove the excess water. Once the watercontent was less than. 23%, the mixture was formed into the shape ofsaddles. The shaped material was dried for two hours at 110° C., thenfired at 1,350° C. in a kiln for 19 hours.

[0021] 1. Accelerated Alkali Corrosion Test:

[0022] Specimens of conventional chemical porcelain saddles,zircon-based ceramic bodies (Ty-Pak™ Heat Sink Media (HSM) from NortonChemical Process Inc., Akron, Ohio) and the alkali-resistant saddles inaccordance with the present invention were buried under pure potassiumcarbonate (melting point 891° C.). The saddles and the K₂CO₃ were heatedto 950° C. and kept at that temperature for 8 hours. This allowed moltenK₂CO₃ to contact the specimens on all sides. The specimens were thencooled, washed with water, and dried. TABLE 3 Corrosion resistance ofporcelain, zircon-based ceramic (Ty-Pak ™ HSM), and alkali-resistantsaddles in accordance with the present invention after 8 hours in moltenK₂CO₃ Weight Weight Weight before Test after Test Change (%) W₁ (gm.) W₂(gm.) ΔW Porcelain Saddles 11.0625  8.1572 −26.3% Ty-Pak ™ HSM  4.6687 4.4869  −3.9% Alkali Resistant Saddles 10.2478 11.0957    0.2%

[0023] As shown in Table 3, porcelain saddles and Ty-Pak HSM lost 26%and 3.9% of their original weights, respectively, due to K₂CO₃corrosion. The weight change for alkali-resistant saddles in accordancewith the present invention was only 0.2%. At high temperatures, K₂CO₃reacted with SiO₂, the main component of porcelain, to form a 1-mm layerof glassy, water-soluble K₂SiO₄. After washing with water, the porcelainsaddles lost 26% of their mass. In contrast, little glaze formed on thealkali-resistant saddles in accordance with the present invention, andthey lost very little mass when washed.

[0024] Physical examination revealed that the size of the porcelainsaddle decreased due to corrosion. The surface of the Ty-Pak™ HSM alsorevealed some corrosion, and surface flaking. In contrast, thealkali-resistant saddles in accordance with the present invention showedvery little change on the surface.

[0025] 2. Ash Build-Up Resistance Test:

[0026] Porcelain saddles, zircon-based ceramic bodies, andalkali-resistant saddles in accordance with the present invention wereburied under wood ash. The wood ash was heated to 870° C. and maintainedat that temperature for 40 hours. The specimens were then washed, driedand weighed. TABLE 4 Build-up of residue on porcelain, zircon-basedceramic (Ty-Pak ™ HSM), and alkali-resistant saddles in accordance withthe present invention after 40 hours in wood ash Weight Weight Weightbefore Test after Test Change (%) W₁ (gm.) W₂ (gm.) ΔW Porcelain Saddles10.6687 11.1168 4.2  Ty-Pak ™ HSM 12.1354 12.2946 1.31 Alkali ResistantSaddles 11.2245 11.2918 0.6 

[0027] As can be seen in Table 4, the alkali-resistant saddles inaccordance with the present invention saddles showed the lowest weightchange, suggesting good resistance to wood ash build-up. Thealkali-resistant saddles did not significantly react with alkalinecomponents of wood ash. The surface of the alkali-resistant saddles wascovered with the deposits of the low-melting-point components in woodash, which washed off easily. In contrast, the SiO₂ in the porcelainsaddles reacted with the wood ash at high temperature. The wood ash wastherefore chemically bound to the porcelain saddles. With a combinationof physical and chemical attachment, the overall weight of the porcelainsaddles increased significantly.

[0028] 3. Thermal Stress Test:

[0029] Porcelain saddles, zircon-based ceramic bodies (Ty-Pak™ HSM), andalkali-resistant saddles in accordance with the present invention wereheated to 870° C. and the temperature maintained for 30 min. The saddleswere then allowed to cool in ambient air. This test was repeated untilthe saddles cracked. TABLE 5 Thermal stress test of porcelain,zircon-based ceramic (Ty-Pak ™ HSM), and alkali-resistant saddles inaccordance with the present invention Breakage Porcelain Alkali aftercycles Saddles Ty-Pak ™ HSM Resistant Saddles  10  3% 0% 0%  20 10% 0%0%  50 100%  0% 0% 100 0% 2%

[0030] The normal operating temperature of RTOs is 850° C. The specimenswere heated to 870° C. for this test, or 20° C. higher than normaloperating temperature. This test showed that alkali-resistant saddles inaccordance with the present invention have good resistance to thermalstress.

[0031] 4. Crushing Strength, Water Absorption, and Porosity:

[0032] The crushing strength of the material was measured by the ASTMC515 standard test method. The water absorption and porosity weremeasured by the ASTM C373 standard test method. The results were asfollows: TABLE 6 Properties of conventional porcelain andalkali-resistant ceramic saddles in accordance with the presentinvention Crushing Strength Water of Saddles (N/m²) Absorption (%)Porosity (%) Porcelain Saddles 1,270 0.2 0.5 Alkali Resistant 2,168 0.41.1 Saddles

EXAMPLE 2

[0033] A batch of alkali-resistant saddles with slightly differentcomposition was prepared and tested the same way as in Example 1.Alkali-Resistant Ceramic Material Chemical Composition of the RawMaterials Raw Material (weight %) (weight %) Light Magnesia 40 MgO 44.6Powdered Talc 27 SiO₂ 38.2 Ceramic Clay 30 Al₂O₃ 10.4 Calcium Carbonate 3 CaO  1.6 Carboxymethyl Cellulose  1 K₂O  1.8 Na₂O  0.9 Fe₂O₃  0.5

[0034] Test Results:

[0035] Accelerated Alkali Corrosion Test: ΔW (%)=0.6%

[0036] Thermal Stress Test: Breakage after 20 cycles=0%

EXAMPLE 3

[0037] A batch of alkali-resistant saddles with slightly differentcomposition was prepared and tested the same way as in Example 1.Alkali-Resistant Ceramic Material Chemical Composition of the RawMaterials Raw Material (weight %) (weight %) Light Magnesia 22 MgO 25.4Powdered Talc 16 SiO₂ 47.2 Ceramic Clay 54 Al₂O₃ 20.1 Barium Carbonate 3 BaO  1.6 Water Glass  4 CaO  2.4 Carboxymethyl Cellulose  1 K₂O  1.0Na₂O  1.2 Fe₂O₃  0.5

[0038] Test Results:

[0039] Accelerated Alkali Corrosion Test: ΔW (%)=1.8%

[0040] Thermal Stress Test: Breakage after 20 cycles=2.0%

EXAMPLE 4

[0041] A mixture of 80 wt % roasted magnesia, 10 wt % ceramic clay, 9.0wt % limestone, 1 wt % water glass, and 1 wt % carboxymethyl cellulosewas ground to a particle size of less than 30 microns. This mixture wasthen applied as a coating onto saddles made of ordinary clay (75 wt %SiO₂, 20 wt % Al₂O₃, 4 wt % K₂O or Na₂O, 2 wt % Fe₂O₃). The saddles werethen dried at 110° C., and fired at 1,300° C. in a kiln.

[0042] Test Results:

[0043] Accelerated Alkali Corrosion Test: ΔW (%)=0.8%

[0044] Thermal Stress Test: Breakage after 20 cycles=10%

[0045] From the above examples and data, it will therefore be clearthat, by controlling the composition as taught in the present invention,it is possible to produce a ceramic material that is substantiallyresistant to attack by alkali salts within the operating environment ofa RTO.

I claim: 1) An alkali-resistant material comprising: to 80% by weight ofMgO; to 50% by weight of SiO₂; to 30% by weight Al₂O₃; and 1 to 10% byweight Fe₂O₃, CaO, and alkali oxides. 2) The alkali-resistant materialof claim 1 wherein the dominant crystalline phases are forsterite andspinel. 3) The alkali-resistant material of claim 1 wherein the alkalioxide is Ka₂O. 4) The alkali-resistant material of claim 1 wherein thealkali oxide is Na₂O. 5) The alkali-resistant material of claim 1wherein the MgO is derived from oxides of magnesium. 6) Thealkali-resistant material of claim 1 wherein the MgO is derived fromtalc. 7) The alkali-resistant material of claim 1 wherein the SiO₂ isderived from clay. 8) The alkali-resistant material of claim 1 whereinthe Al₂O₃ is derived from clay. 9) A process for the production of analkali-resistant ceramic body comprising the steps of: a) grindingtogether a mixture comprising 0% to 50% by weight of light magnesia, 0%to 85% by weight of roasted magnesia, 10% to 60% by weight of ceramicclay, 0% to 15% by weight of limestone, 0% to 3% by weight of waterglass, 0% to 3% by weight of carboxymethyl cellulose, 0% to 30% byweight of talc, and 0% to 30% by weight of calcium or barium carbonateto a particle size of less than 50 microns for 80% of the particles; b)mixing the ground mixture with water to produce a paste containing lessthan 30% by weight of water; c) shaping the paste to a desired shape; d)drying the shaped product at a temperature greater than 100 degreescentigrade to make it suitable for firing in a kiln; and e) firing thedried shaped product in a kiln at 1,250 to 1,450 degrees centigrade. 10)A process for coating a conventional ceramic body with analkali-resistant ceramic coating comprising the steps of: i) preparingan alkali resistant ceramic coating by grinding together 75% to 85% byweight of roasted magnesia, 5% to 10% by weight of ceramic clay, 0% to15% by weight of limestone, 0% to 2% by weight of water glass, and 0.5%to 2% by weight of carboxymethyl cellulose to a particle size of lessthan 40 microns for 80% of the particles; ii) mixing the ground mixturewith water to produce a paste containing less than 30% by weight ofwater; iii) applying a coating of the alkali resistant material paste tothe surface of the conventional ceramic body; iv) drying the coatedceramic body at a temperature greater than 100 degrees centigrade; andv) firing the dried coated ceramic body in a kiln at 1,200 to 1,400degrees centigrade. 11) An alkali-resistant ceramic body comprising: to80% by weight of MgO; to 50% by weight of SiO₂; 5 to 30% by weightAl₂O₃; and 1 to 10% by weight Fe₂O₃, CaO, and alkali oxides. 12) Thealkali-resistant ceramic body of claim 11 wherein the dominantcrystalline phases are forsterite and spinel. 13) The alkali-resistantceramic body of claim 11 wherein the alkali oxide is Ka₂O. 14) Thealkali-resistant ceramic body of claim 11 wherein the alkali oxide isNa₂O. 15) The alkali-resistant ceramic body of claim 11 wherein the MgOis derived from oxides of magnesium. 16) The alkali-resistant ceramicbody of claim 11 wherein the Mgo is derived from talc. 17) Thealkali-resistant ceramic body of claim 11 wherein the SiO₂ is derivedfrom clay. 18) The alkali-resistant ceramic body of claim 11 wherein theAl₂O₃ is derived from clay. 19) The alkali-resistant ceramic body ofclaim 11 wherein the porosity is less than 5% by volume. 20) Thealkali-resistant ceramic body of claim 11 wherein the water absorptionis less than 5% by weight 21) The alkali-resistant ceramic body of claim11 wherein the compressive strength of the ceramic material is greaterthan 2×10⁸ Newtons per square meter. 22) The alkali-resistant ceramicbody of claim 11 wherein the loss in weight of the alkali-resistantceramic body is less than one percent when exposed to molten potassiumcarbonate. 23) The alkali-resistant ceramic body of claim 11 wherein thegain in weight of the alkali-resistant ceramic body is less than onepercent when exposed to wood ash at greater than 800 degrees centigrade.24) A ceramic body coated with an alkali-resistant material, the alkaliresistant coating comprising: to 80% by weight of MgO; to 50% by weightof SiO₂; 5 to 30% by weight Al₂O₃; and 1 to 10% by weight Fe₂O₃, CaO,and alkali oxides. 25) The ceramic body of claim 24 wherein the dominantcrystalline phases of the alkali resistant coating are forsterite andspinel. 26) The ceramic body of claim 24 wherein the porosity of thealkali resistant coating is less than 5% by volume. 27) The ceramic bodyof claim 24 wherein the water absorption of the alkali resistant coatingis less than 5% by weight. 28) The ceramic body of claim 24 wherein thealkali oxide is Ka₂O. 29) The ceramic body of claim 24 wherein thealkali oxide is Na₂O. 30) The alkali-resistant material of claim 24wherein the MgO is derived from oxides of magnesium. 31) Thealkali-resistant material of claim 24 wherein the MgO is derived fromtalc. 32) The alkali-resistant material of claim 24 wherein the SiO₂ isderived from clay. 33) The alkali-resistant material of claim 24 whereinthe Al₂O₃ is derived from clay.